This application is based upon and claims the benefit of priority under 35 U.S.C. xc2xa7119(a)-(d) based on Japanese Patent Application No. 2000-015653, filed Jan. 25, 2000.
The present invention relates to a semiconductor device such as a semiconductor integrated circuit, a method of processing a copper film to be used for wirings for a semiconductor device such as a semiconductor integrated circuit and a method of producing a wiring structure for such a semiconductor device.
Today, copper or a material essentially consisting of copper is used for wirings for a semiconductor device such as a semiconductor integrated circuit. In such a case, copper atoms are diffused into an insulating film to enter the semiconductor substrate, which may result in malfunction of transistors. To prevent it, a barrier metal layer of a titanium nitride, tantalum nitride, tungsten nitride or the like is often formed around the copper film to isolate the copper film from the insulating film.
As shown in FIG. 13A which is a cross-sectional view showing the structure of a semiconductor substrate, wirings formed on a semiconductor substrate are normally buried in wiring grooves in an insulating film. An inter-layer insulating film 101 is formed of silicon oxide on a semiconductor substrate 100 of silicon or the like where a semiconductor element (not shown) such as an integrated circuit is formed, and wiring grooves are formed in the surface of this interlayer insulating film 101. A barrier metal layer 102 of TiN, TaN or the like is formed on the side wall of each wiring groove and a copper film 103 or an alloy film essentially consisting of copper is buried in the region surrounded by the barrier metal layer 102.
Apparently, the conventional wiring may cause copper in the wiring portion to be diffused into the inter-layer insulating film 101 from a portion where there is no barrier metal, i.e., from above, thereby adversely affecting the semiconductor element formed on the semiconductor substrate 100. As the surfaces of the inter-layer insulating film 101 and the copper film 103 buried therein are planarized by CMP or the like, the pattern edges may not be detected at the time of implementing lithography, which may lead to deviation in the wiring pattern.
Those shortcomings can be dealt with by selling back the wiring portion or the copper film 103 from the surface of the inter-layer insulating film by etching (FIG. 13B). Making such a shape can provide accurate pattern alignment.
There may be a case where a barrier metal layer 104 may be buried in the recessed portion to cover the copper film 103 as shown in FIG. 13C. This structure has an advantage of suppressing the diffusion of copper from above the wiring. Further, forming the cap layer of a conductive material prevents copper from being placed in the etching environment at the time a via wiring (contact wiring) to the over-lying wiring is formed. This can reduce the possible occurrence of corrosion or etching-originated wiring disconnection.
The above-described conventional wiring structures are formed by two methods: wet etching and dry etching. Dry etching includes anisotropic etching called RIE (Reactive Ion Etching) and isotropic etching called CDE (Chemical Dry Etching), both of which can etch copper. Those dry etching processes often use CF-based gas as the source gas, which adversely affect the environment of the earth. Further, because a by-product is deposited after etching, a wet process for removing the by-product is often performed so that dry etching is disadvantageous over wet process in the number of steps as well as the cost.
In this aspect, attention is being paid to wet etching. Copper is hardly dissolved in a weak acid which has a weak oxidizing property, such as hydrochloric acid, hydrofluoric acid, dilute sulfuric acid, acetic acid, hydrocyanic acid, or the like, however it is etched by an acidic chemical solution which has an oxidizing property. Specifically, such acidic chemical solutions include thermal concentrated sulfuric acid, nitric acid, nitrous acid, phosphoric acid and the like. Copper is also soluble in an acid solution which is formed by mixing acid with aqueous hydrogen peroxide, ozone or oxygen, such as hydrochloric acid+aqueous hydrogen peroxide, hydrochloric acid+aqueous ozone or hydrofluoric acid+aqueous hydrogen peroxide. Further, copper is also etched by a material which forms a soluble complex with copper, such as ammonia, a material having an amino group (ethylene diamine or the like), cyanide (KCN or the like). Etching is often accelerated by mixing aqueous hydrogen peroxide or the like with those materials to provide an oxidizing property.
Normally, a mixed solution of aqueous ammonia and aqueous hydrogen peroxide and a mixed solution of hydrochloric acid and aqueous hydrogen peroxide are used as cleaning chemical solutions and are respectively called SC1 and SC2. The concentrations of aqueous ammonia, hydrochloric acid and aqueous hydrogen peroxide available on the market are often about 20 to 35%, and SC1 and SC2 are usually mixed with pure water in the volume ratio of about 1:1:5 to 1:1:7. As copper is dipped into the solution mixed under the above condition, it is etched.
When copper is etched with SC1 or SC2 under the aforementioned condition, however, the copper surface having a metallic gloss becomes whitened and loses gloss. Etching copper with any of the aforementioned chemical solutions such as nitric acid and thermal concentrated sulfuric acid also causes the whitening phenomenon. This phenomenon becomes more prominent unless the temperature of the solution is raised. Whitening of the surface of copper occurs as etching roughens the copper surface. If this copper is used for wirings and the surface roughness becomes larger, a variation in film thickness or the surface scattering may increase the substantial resistance or result in contact failure to the over-lying wiring. It is therefore necessary to perform etching under such a condition as not to roughen the copper surface as much as possible.
Another way of etching copper is to oxidize a copper film and then remove the copper oxide using acid or the like. For example, Jpn. Pat. Appln. KOKAI Publication No. 2-306631 proposes a method of carrying out implantation of oxygen ions in a copper film and then annealing the resultant structure or subjecting the structure to an oxygen plasma treatment to form a copper oxide, and then etching the copper oxide with diluted sulfuric acid or acetic acid. In addition, Jpn. Pat. Appln. KOKAI Publication No. 10-233397 proposes a method of placing a copper film in the environment of oxygen (O2) or ozone (O3) in a diffusion furnace, an RTA furnace or an oven at room temperature or higher to thereby form a copper oxide film and then removing the copper oxide film by wet etching using diluted hydrochloric acid or diluted sulfuric acid, or dry etching or CMP. Those schemes however often cause copper to have a rough surface after etching. Particularly, this tendency becomes more noticeable as the temperature is increased to make the oxide film thicker.
Accordingly, it is an object of the present invention to provide a method of forming a copper oxide film, a method of etching a copper film, a method of fabricating a semiconductor device, and a semiconductor device, which suppress roughening of the surface of a copper film after performing an etching process of oxidizing the copper film and removing the copper oxide with acid, alkali or the like, and allow the etching process to be implemented in fewer steps and quickly at a high precision.
This invention is characterized in that, in forming a copper wiring, copper is placed in a mixed solution (SC1) of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 8 to 10, thereby forming a copper oxide film including an ammonia complex on the surface of the copper, after which the copper oxide film is selectively etched with acid having a weak oxidizing property such as diluted hydrochloric acid or alkali such as diluted aqueous ammonia. To shorten the etching time, after copper is dipped in the mixed solution of pH of 8 to 10 to form an oxide film, the copper is dipped in SC1 which has been adjusted to have pH of 10 to 11 which would normally etch copper, thereby forming a thicker copper oxide film including an ammonia complex, which is then selectively etched with acid having a weak oxidizing property such as hydrochloric acid or alkali such as diluted aqueous ammonia. Copper oxide film can be selectively etched even with liquid which dissolves copper by forming complex with copper, though the liquid is neutral as an aqueous solution of neutral amino acid such as glycine or alanine.
Such formation of an oxide film and such etching can permit copper to be etched without roughening the copper surface which has conventionally been difficult to achieve, can ensure quick oxidation and etching with a safe and inexpensive chemical solution. As a result, a barrier metal layer to be coated on the surface of the wiring structure is formed stably.
To achieve the above object, according to one aspect of this invention, there is provided a method of forming a copper oxide film which comprises the step of forming a copper oxide film including an ammonia complex by causing a mixed solution of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 8 to 10 or pH of 9 to 10, to contact a surface of a copper film.
According to another aspect of this invention, there is provided a method of forming a copper oxide film which comprises the steps of forming a copper oxide film including an ammonia complex by causing a mixed solution of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 8 to 10 or pH of 9 to 10, to contact a surface of a copper film; and exposing the copper film having the copper oxide film formed on the surface thereof to a mixed solution of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 10 to 11.
According to a further aspect of this invention, there is provided a method of forming a copper oxide film which comprises the steps of forming an oxide film on a surface of a copper film using aqueous hydrogen peroxide; and forming a copper oxide film including an ammonia complex by placing the copper film having the oxide film formed thereon in a mixed solution of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 10 to 11.
In a case that only aqueous hydrogen peroxide or a mixed solution of aqueous hydrogen peroxide and aqueous ammonia, which has been adjusted to have pH of 8 to 10 or pH of 9 to 10, is first caused to contact a surface of a copper film to form a copper oxide film, and the copper film having copper oxide film is finally exposed to a mixed solution of aqueous ammonia and aqueous hydrogen peroxide, which has been adjusted to have pH of 10 to 11, pH may be varied intermittently in a multi-stage manner or continuously in the course of performing the method of forming a copper oxide film.
According to a still further aspect of this invention, there is provided a method of etching a copper film which comprises the steps of forming a copper oxide film including an ammonia complex on a surface of the copper film by using the copper oxide film forming method as recited in any one of the first to third aspects; and selectively removing the copper oxide film from the copper film. The copper oxide film may be removed with acid or alkaline.
According to a yet still further aspect of this invention, there is provided a method of fabricating a semiconductor device which comprises the steps of burying a copper film to be a wiring or a contact wiring in a wiring groove or a contact hole formed in a surface of an insulating film formed on a semiconductor substrate, or in both the wiring groove and the contact hole; forming a copper oxide film including an ammonia complex on a surface of the copper film by using the copper oxide film forming method as recited in any one of the first to third aspects; and selectively removing the copper oxide film from the copper film.
The surface of the copper film from which the copper oxide film has been removed may be etched deeper at a region closer to the wiring groove or the contact hole.
In the semiconductor device fabricating method, a barrier metal layer may be intervened between the wiring groove or the contact hole and the buried copper film or between the wiring groove and contact hole and the buried copper film.
The semiconductor device fabricating method may further comprise the step of forming a barrier metal layer on the copper film after removing the copper oxide film from the copper film.
In the semiconductor device fabricating method, the barrier metal layer intervened between the wiring groove or the contact hole and the buried copper film or between the wiring groove and contact hole and the buried copper film and the barrier metal layer formed on the copper film may be made of different materials.
The semiconductor device fabricating method may further comprise the step of placing the surface of the copper film from which the copper oxide film has been removed in aqueous ammonia.
In the semiconductor device fabricating method, with the semiconductor substrate being rotated at a speed of 1000 rpm to 1600 rpm, the surface of the copper film may be placed in the aqueous ammonia.
According to another aspect of this invention, there is provided a method of fabricating a semiconductor device, comprising the steps of: filling a wiring groove or a contact hole formed in an insulating film formed on a semiconductor substrate with wiring metal by depositing the wiring metal in the wiring groove or the contact hole; exposing the insulating film by polishing the wiring metal; cleaning the semiconductor substrate; and carrying out recessing etching on a surface of the wiring metal buried in the wiring groove or the contact hole, wherein chemical solutions used in at least two steps of the polishing step, the cleaning step and the recessing etching step have the same main component.
According to a further aspect of this invention, there is provided a method of fabricating a semiconductor device comprising the steps of: depositing metal or metal compound on a semiconductor substrate; and etching out an unnecessary portion of the metal or metal compound by etching; wherein the step of depositing metal or metal compound includes a plating step, component in plating solution used in the plating step forming salt or complex with a component to be plated is the same as a main component of chemical solution used in the etching-out step. In the semiconductor device fabrication method, the step of depositing the wiring metal may be a plating step and a main acidic component in plating solution used in the plating step may be the same as a main component of the chemical solution. The main oxidizing agent in the chemical solution may be hydrogen peroxide or ozone. The main acidic component in the chemical solution may be sulfuric acid or hydrocyanic acid. The semiconductor device fabrication method may further comprise a step of removing oxidizing agent contained in the chemical solution after the step of etching-out an unnecessary portion, a step of making metal ion concentration in the chemical solution approximately equal to metal ion concentration in the plating solution, and a step of using the chemical solution where the oxidizing agent has been removed as the plating solution.
Also, according to anther aspect, there is provided an apparatus of fabricating a semiconductor device used for the above method of fabricating a semiconductor device, comprising means for removing oxidizing agent contained in the chemical solution; means for making metal ion concentration in the chemical solution approximately equal to metal ion concentration in the plating solution; and means for using the chemical solution where the oxidizing agent has been removed as the plating solution.
According to another aspect, there is provided a semiconductor device comprising a semiconductor substrate; a metal film buried in a wiring groove, a contact hole, or the wiring groove and the contact hole formed on an insulating film formed on the semiconductor substrate; and a barrier metal layer formed on the metal film buried in the wiring groove, the contact hole, or the wiring groove and the contact hole so as to cover a surface of the metal film, wherein a surface of the metal film is etched deeper at a region closer to a side wall of the wiring groove or the contact hole. The metal film may be buried in the wiring groove or the contact hole via barrier metal. The barrier metal formed so as to cover the surface of the metal film may have a structure where it has been buried in the wiring groove or the contact hole. That is, the closer to the side wall of the wiring groove, the greater the etching amount of a surface of such a metal film as a copper film, and the surface of the metal film has a cross section with shoulders of the wiring dropped.
Therefore, the closer to a side wall of the wiring groove the barrier metal layer formed on the copper film is, the thicker the barrier metal layer becomes. This shape is frequently advantageous when this invention is worked out. For example, when the contact hole for forming a contact wiring is formed on the barrier metal layer, the etching area where the contact hole is to be formed partly cut into an inter-layer insulating film due to misalignment in some cases. When the inter-layer insulating film which covers the barrier metal layer of the under-layer wiring is etched under such a situation, etching of the inter-layer insulating film progresses because the etching rate of such an inter-layer insulating film as a silicon oxide film is greater than that of such a metal film as a copper film, so that portion is greatly etched. In the case of the shoulders of the wiring dropped, the diameter of a deep portion Is larger than that in a case of a flat surface. That is, the aspect ratio of this portion is low. Thus, it is easy to deposit a barrier metal layer and deposit a seed metal film as a copper film. Forming the contact hole is not limited to a case that a barrier metal layer is buried.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.